1. Field of the Invention
The invention relates generally to the field of marine seismic surveying. More particularly, the invention relates to techniques for determining whether one or more air guns used in a marine seismic energy source array suffer from operational faults, such as air leaks.
2. Background Art
In marine seismic surveys, a seismic energy source is used to generate seismic energy in the form of acoustic pulses or waves in a body of water such as a lake or the ocean. The seismic energy travels downward in the water, through the water bottom, and through the Earth formations underlying the water bottom. Part of the energy passing through the Earth formations underlying the water bottom is reflected upward therefrom at acoustic impedance boundaries in the Earth formations. The upward traveling energy is detected by sensors such as hydrophones towed in one or more streamer cables disposed near the water surface, or by sensors disposed in cables along the water bottom. The sensors convert the detected energy to electrical or optical signals. The electrical or optical signals are then conditioned and interpreted to provide information both as to the composition and the structure of the various subsurface Earth formations. Such information is used particularly to determine the possibility that such Earth formations may contain mineral deposits such as hydrocarbons.
Several different types of seismic energy sources have been used in the past to produce seismic energy in a form required in marine seismic surveys. For example, explosives have been used as a marine seismic energy source, but explosives are dangerous and are considered ecologically unacceptable for such purposes. Another type of marine seismic energy source, called a gas gun, includes detonating combustible gases in a chamber and then expelling the resultant gas charge into the water to produce the seismic energy. Certain risks are associated with the use of such gas guns, and as such gas guns are not widely used.
The most frequently used marine seismic energy source at present is known as an “air gun.” In an air gun, a charge of compressed air or inert gas under high pressure, on the order of 2000-6000 pounds per square inch (130 to 400 bars), is supplied to a chamber within the gun. When the air gun is actuated or “fired”, a valve rapidly opens to discharge the pressurized air or gas from the chamber into the water. The valve then closes and the chamber is repressurized with air or gas from a source such as a compressor. Such firing can be repeated as often as desired and within the capacity of the source to repressurize the chamber. The air or gas discharges in a bubble or plurality of such bubbles in the water.
A single air gun produces a seismic pulse having acoustic energy content with respect to frequency (the source “signature”) related to a complex pressure interaction between the air bubbles and the water that causes the bubbles to oscillate as they float toward the water surface. Such interaction can produce extraneous bursts of seismic energy following the initial energy burst. The amplitude and periodicity of these bubble-generated extraneous bursts depend on, among other factors, the depth of the gun in the water and the size of the pressurized air chamber in the gun. It is therefore known in the art to use an array of air guns having various different chamber sizes, and firing such guns contemporaneously or in a preselected timing sequence. Such firing of an array of air guns provides several advantages over firing a single air gun. First, the total amount of energy being imparted into the Earth's subsurface for each seismic “shot” is increased. In addition, the different chamber sizes for the various guns will produce different bubble responses, causing the bubble responses to tend to cancel each other. The directivity of the energy source toward the water bottom can be improved, because other than directly below the source array, some frequencies in the seismic energy will be attenuated by the spatial distribution of the guns in the array. Design considerations for air gun arrays to attain certain spectral and directivity characteristics are disclosed in marine seismic surveys, a seismic energy source is used to generate seismic energy in the form of acoustic pulses or waves in a body of water such a lake or the ocean. The seismic energy travels downward in the water, through the water bottom, and through the Earth formations underlying the water bottom. Part of the energy passing through the Earth formations underlying the water bottom is reflected upward therefrom at acoustic impedance boundaries in the Earth formations. The upward traveling energy is detected by sensors such as hydrophones towed in one or more streamer cables disposed near the water surface, or by sensors disposed in cables along the water bottom. The sensors convert the detected energy to electrical or optical signals. The electrical or optical signals are then conditioned and interpreted to provide information both as to the composition and the structure of the various subsurface Earth formations. Such information is used particularly to determine the possibility that such Earth formations may contain mineral deposits such as hydrocarbons.
Adjusting the number, position, operating pressure and chamber volumes of the air-guns in an array changes its signature. The array can thus be tailored to the requirements of a particular survey depending on the depth and geologic nature of the target interface of interest and the overlying rocks. However, in the event of equipment failure, the signature of the array will change. If the detrimental effect on the signature of the whole array is severe, acquisition must cease while the problem is rectified. One such failure mode is leakage of the high-pressure air that supplies the guns into the water column. The effects of such air-leaks can include suppression of the reflection of the wave-field from the water surface near the array for guns in the vicinity of the leak and extra energy from oscillation of the bubbles released into the water column.
Thus, a need exists for a method to identify air-gun faults as early as possible. Such identification will allow planned preventative maintenance to be carried out at a convenient time, which is preferable to a forced cessation of acquisition when a serious fault occurs.